1. Field of the Invention
The present invention relates to an optical fiber wavelength filter employed for selecting an optical wavelength in an optical fiber communication or an optical measurement system, and a manufacturing method for the same.
2. Description of the Related Art
The U.S. Pat. No. 5,706,379 has disclosed an optical fiber wavelength filter that employs a multilayer dielectric film filter formed on a glass substrate; FIG. 4 is a cross-sectional view showing the optical fiber wavelength filter.
Optical fibers 21 and 22 are inserted into ceramic ferrules 23 and 24 and bonded therein to prepare a pair of ground ferrules with optical fibers. Then, a glass substrate filter 25 composed of a glass substrate and a multilayer dielectric film formed on the surface thereof is placed between the foregoing pair of ferrules 23 and 24 with optical fibers, and the optical axes of the ceramic ferrules 23 and 24 are aligned using an aligning sleeve 26 before securing them with an adhesive agent.
This prior art has been posing a problem in that the use of the glass substrate filter 25, in which thickness t is normally a few hundred .mu.m (.gtoreq.300 .mu.m), causes transmitted light to be refracted by the glass substrate or mode field diffusion to take place. The optical axis of the refracted light is inevitably displaced from the central axis of the ferrules.
The mechanical central axes of the ceramic ferrules with optical fibers are aligned by the inside diameter of the aligning sleeve and the misalignment of the optical axis of the transmitted light caused by the refraction can hardly be corrected. The results of the experiments carried out by the inventor have revealed that, when thickness t of the glass substrate filter is 300 .mu.m, the measured value of the insertion loss is approximately 7 dB mainly due to the dislocated optical axis, or when the thickness is 500 .mu.m, the insertion loss is approximately 9 dB. This means that it is difficult to control the insertion loss in this wavelength filter. In addition, a thermosetting adhesive agent is usually used in manufacturing the optical fiber wavelength filters employing the aforesaid aligning sleeves; this type of adhesive agent typically requires at least a few tens of minutes to completely harden.
The U.S. Pat. No. 5,234,772 has disclosed a dielectric multilayer film filter and a manufacturing method for the same. The optical fiber wavelength filter employs a filter composed of a dielectric multilayer film formed on a fluorinated polyimide thin film (hereinafter referred to as "thin-film filter"). The filter produced according to this process has an extremely thin wavelength filter owing to the use of the fluorinated polyimide thin film; hence, this filter is free from the deterioration in the optical properties resulting from the use of the thick glass substrate filter mentioned above.
FIG. 5 is a top plan view of an optical fiber wavelength filter employing the conventional thin-film filter; and FIG. 6 is a longitudinal sectional view of a wavelength filter employing the conventional thin-film filter.
Optical fibers 35 and 36 (the two optical fibers are not cut at the beginning) are secured to a substrate 30 by an adhesive agent. Then, the optical fibers are cut and a groove 31 is formed across, then a thin-film filter 32 is inserted in the groove 31 and secured therein by an adhesive agent. The U.S. patent has also disclosed an example wherein a zirconia ferrule is used in place of the substrate 30, optical fibers being attached and bonded thereto, and the zirconia ferrule or a stainless flange is provided with a groove wherein the thin-film filter is disposed.
It is necessary to provide the substrate 30, the substrate with optical fibers, the zirconia ferrule, or the like with the groove wherein the thin-film filter is disposed; it is not easy, however, to precisely form the groove having a width of about a few tens of .mu.m. The variations in the machining dimension of the groove width directly affect the properties of the wavelength filter and lead to the variations in the optical properties such as insertion loss of the wavelength filter. Furthermore, the end surfaces of the optical fibers that are cut apart at the time of making the groove for holding the thin-film filter cannot be ground, so that the irregular reflection of light may take place on the connection end surfaces with consequent deterioration in optical properties.
Thus, the conventional optical fiber wavelength filters have many problems in the manufacturing process or optical properties.